1. Field of the Invention
The present invention relates to a semiconductor device.
In this specification, a “semiconductor device” refers to a general device that can function by utilizing semiconductor characteristics; an electro-optical device, a semiconductor circuit, and an electronic device can be included in the category of the semiconductor device. A device including a semiconductor device is regarded as a semiconductor device.
2. Description of the Related Art
A transistor is used in a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). As materials of the semiconductor that can be used in the transistor, silicon-based semiconductor materials have been widely known, but oxide semiconductors have been attracting attention as alternative materials.
For example, a transistor including an amorphous oxide semiconductor layer containing indium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document 1.
Techniques for improving carrier mobility by employing a stacked structure of an oxide semiconductor layer are disclosed in Patent Documents 2 and 3.
It is known that a transistor including an oxide semiconductor layer has an extremely small leakage current when the transistor is off. For example, a low-power-consumption CPU utilizing such a small leakage current characteristic of a transistor including an oxide semiconductor layer is disclosed (see Patent Document 4).